In the wireless ad hoc communication system, it is necessary to enable a plurality of wireless terminals (may simply be referred to as terminals) present in a communication area to communicate with one another. Hence, the terminals are directly interconnected to communicate with one another in an ad hoc mode of a wireless local area network (LAN), or the directly interconnected terminals relay other terminals' signals (packets) to communicate with one another. Thus, used frequencies are often similar among the terminals. However, a capacity (bandwidth) for one radio frequency channel (may simply be referred to as channel) is limited. Thus, when the number of simultaneously operated peripheral terminals increases, a capacity becomes short, causing a transmission delay or a signal collision. In consequence, performance is lowered.
In a mobile communication system, when all base stations use the same frequency, there arise problems that not only a capacity for each channel becomes short but also a radio interference occurs. Thus, a communication area is generally divided into zones (cells) of certain sizes or shapes, and channels are arranged to prevent interferences between adjacent cells (e.g., refer to “Basics of Mobile Communications” by the Institute of Electronics and Communication Engineers, under the editorship of Okumura and Shinji).
Thus, as illustrated in FIG. 1, a communication area in the wireless ad hoc communication system is divided into a plurality of cells, and a channel is allocated to each cell. In this case, while terminals of the cells using the same channel can communicate with each other, terminal shaving different channels allocated thereto cannot communicate with each other even when the terminals are located in positions near to each other.
To solve this problem, for example, each terminal may include two wireless interfaces, and one wireless interface may use an allocated channel of each cell while a fourth channel for relay other than three channels allocated for each cell may be allocated to the other wireless interface.
However, in the case of a wireless LAN (IEEE802.11b/11g) of a bandwidth of 2.4 GHz, only three channels (e.g., radio frequency channels 1 ch, 6 ch, and 11 ch) can be used without any mutual interferences, and no relay channel can be allocated.
Even when a relay channel can be allocated as in the case of a wireless LAN (IEEE802.11a) of a bandwidth of 5 GHz, whether a next transfer destination of a transmitted packet is the same cell or a different cell has to be judged, necessitating cumbersome processing associated with ad hoc routing.
Additionally, in the wireless ad hoc communication system, no wireless station (e.g., equivalent to a base station of a mobile communication system) that has a management function of asking for information of cells and channel arrangement is included. Thus, each terminal has to hold such information. When a terminal is mobile, management information of all communication areas to which the terminal is likely to move has to be provided, increasing the amount of information to be held. Thus, this case is not realistic.
The following are related arts to the invention.
[Patent document 1] Japanese Patent Laid-Open Publication No. JP 2005-101716
[Patent document 2] Japanese Patent Laid-Open Publication No. JP 2005-303828